1. Field of the Invention
The invention relates generally to the field of digital data communications and to a communications system having integrated therein both packet-switching and message switching (store-and-forward) communication techniques for enabling communication between a plurality of dissimilar data input terminals, such as various facsimile terminals, visual display units, teletype terminals, printer terminals, TWX or Telex, word processing equipment, and computer terminals. More particularly, the invention relates to a communications system and method embodying a novel data conversion technique for enabling a system user to communicate via any digital input thereto with any other user or combination of users at any other digital access points to the system, regardless of difference in modulation techniques, character codes, data protocol or "handshake", synchronization and, in the case of facsimile, any of the above plus variation in speed and drum diameter, thereby enabling otherwise incompatible terminals to communicate with each other with a system-wide compatible protocol to enable effective information exchange. The invention further, and more particularly in a preferred embodiment thereof, relates to an improved facsimile transmission and communication system incorporating novel data conversion, compression and network control techniques for communication between dissimilar facsimile units.
2. Description of the Prior Art
New data transceiving devices are increasingly being developed and implemented into an increasing number of teleprocessing terminals, putting ever increasing demands upon existing digital data switching communications networks. It is essential that such terminals have the ability to communicate with each other; however, due to the variety of codes, modulation techniques, data protocols and other technological differences between the various terminals produced by an ever increasing number of manufacturers, many of these terminals are inhibited from communication with each other due to these incompatibilities. Costly communication processors and special software programs have been used in the past to achieve the code and other data conversions necessary to permit inter-terminal communications. The present invention overcomes these problems and allows a wide variety of data terminals of various protocols, codes, and modulation techniques to communicate with each other directly, without the need for individual complex interface equipment at each terminal location.
While the present invention is applicable to communication between a wide variety of teleprocessing equipment as described herein, it is particularly suitable for providing a compatible means of communication between a wide variety of dissimilar and otherwise incompatible facsimile equipment. The problem of facsimile communication is particularly affected by incompatibilities in resolution, modulation, handshaking procedures and drum sizes existing in the various equipment produced by competitive manufacturers.
A stored program facsimile control system of the prior art is described by U.S. Pat. No. 3,751,582, in which system a plurality of different types of facsimile devices may be interfaced. This is accomplished by varying the processor control operation with various stored programs, thereby enabling compatible operation between facsimile transceivers having different operating characteristics. While the described prior art system provides compatible communication between two facsimile units, the present invention provides compatible communication between a great number of facsimile terminals simultaneously, for example, between one hundred twenty terminals, without delay, data loss or complex protocols. Both multi-terminal communication capability and terminal compatibility is provided by the present invention. Additionally, the described prior art system requires an operator to select the requisite program from storage, whereas, in accordance with the present invention, the programs utilized in the course of providing compatible operation between dissimilar facsimile units are stored in the system memory and automatically selected based upon a table reference or look-up once the originating and final destination codes indicative of machine type are received, thereby obviating operator error.
Another prior art facsimile transmission system is described in U.S. Pat. No. 3,739,338, wherein apparatus is provided for enabling a data device to be coupled to a communication channel to permit the communication of data over the channel such as a conventional telephone system. U.S. Pat. No. 3,920,896 describes a switched communication system including store-and-forward means for enabling transmission between various facsimile devices. U.S. Pat. No. 3,646,256 describes a facsimile transmission system wherein the rate of transmission is determined by the content of the transmitted information. U.S. Pat. No. 3,558,811 describes an interface adapter unit for converting facsimile graphic information signals to a copy. U.S. Pat. No. 3,714,377 describes a store-and-forward speed changer for off-line transmission of binary data. U.S. Pat. No. 3,876,825 describes a facsimile conversion unit which digitizes the facsimile signal for subsequent synthesis into a four level compressed bandwidth baseband signal. U.S. Pat. No. 3,830,962, a graphic data processor interface, adapter unit for interconnecting a facsimile graphic communications system with a central processing unit. U.S. Pat. No. 3,849,595 describes a facsimile signal transmission system using both AM and phase modulation, wherein the phase modulations are based on the binary values of the input facsimile signal. U.S. Pat. No. 3,868,477 describes a circuit for providing scanner baseband compensation by converting the scanner signal to black or white, based on threshold detection and rate of change detection. U.S. Pat. No. 3,392,232, a facsimile transmission system, describes the use of an acoustic coupler for the transmission of facsimile signals. U.S. Pat. No. 3,292,148 describes a method and apparatus for sensing and identifying information-bearing indicia. U.S. Pat. No. 3,831,091 describes a facsimile transceiver utilizing a time-shared circuit for performing a variety of functions. U.S. Pat. No. 3,614,319 describes a facsimile transmission system via telephone wherein initiation of the data handshake exchange after initialization results in synchronization of the transmitting and receiving stations.
Various data compression techniques have been developed in the prior art. Compression of bit patterns of adjacent scan lines in a two bit pattern sometimes referred to as the "delta coding method", is described in U.S. Pat. No. 3,804,975. "Run-length encoding" utilizes a compression algorithm which eliminates redundancy in transmitted black and white data by encoding an entire line, a portion thereof or a number of succeeding lines of all black or all white as a variable length code, rather than repetitiously encoding each line segment. Such encoding is described by Huffman, D. A., "A Method for the Construction of Minimum Redundancy Codes Proc. Inst. Radio Engineers", 40 (1952) page 1098 Bandwidth compression, the reducing of the frequency of the signal components comprising the representation of the facsimile data serves to reduce transmission time, particularly when the transmission medium is a limited bandwidth telephone line. Such a bandwidth compression technique is described by U.S. Pat. No. 3,761,610. A dual-line compression algorithm is described by U.S. Pat. No. 3,916,095 wherein two lines of scanned data are treated simultaneously for purposes of encoding and decoding. The data compression techniques of the present invention may utilize a compression algorithm such as a dual-line encoding algorithm similar to that of the referenced patent, however, with the key distinction of incorporating the compression technique within the present packetized system. In other words, compression is achieved throughout the entire facsimile page, but the compressed data is transmitted as packets for ultimate expansion and reassembly.
A description of various switched communication systems having store-and-forward capabilities are described with references thereto in the aforementioned U.S. Pat. No. 3,920,896 and references made thereto for further detail with regard to such prior art systems.
Store-and-forward data transmission, sometimes referred to as message switching may be defined as a mode of transmission wherein data message or portions thereof are accumulated, stored and retransmitted on a scheduled or priority basis as desired and in accordance with channel and/or equipment availability to the next desired location, thereby maximizing the efficiency of transmission in accordance with a predetermined priority structure for message flow. Such storing may be for a time duration of several hours or more when required.
Packet switching is a priority mode of data transmission, particularly useful in computer-to-computer communication in that retransmission occurs almost instantaneously, providing a virtual channel for a computer terminal with a dedicated input port without time consuming circuit switching and connection procedures. In packet switching systems, data is subdivided into "electronic envelopes" called packets, each packet consisting of a predetermined number of data bytes, together with synchronization and other identifying data as a header, and an error checking code. The composition of the individual packets is described in greater detail hereinafter. The data packets are integrated into the overall data stream of the communication network in accordance with the present invention together with other data transmissions in the store-and-forward mode in a time division multiplexed scheme. The packets are transmitted from their source terminal through the various network switching sites or nodes to their intended destination, at which point they are reassembled as required into the original data messages. The data content of the individual packets is variable and dynamic and capable of independent routing through the communication network before the multiple packets are reassembled into the final message. For descriptive purposes, a typical message may consist of a data sequence of approximately one hundred fifty packets of information.
Packet switching systems of the prior art are described by L. Pouzin (Reseau Cyclades), A Proposal for Interconnecting Packet Switching Networks; Eurocomp Brunnel University, London, May 1974; "The Interface Message Processor for the ARPA Computer Network", Spring Joint Computer Conference, May 7, 1970 by Heart, F. E., et al; Presentation and Major Design Aspects of Cyclades Computer Network, Pouzin, L. 3rd Data Communications Symposium, April 1973; Datapac Standard Network Access Protocol, The Computer Communications Group, Trans-Canada Telephone System, Mar. 31, 1976; CTNE's Packet Switching Network, Alarcia G. Herrera, Second International Conference on Computer Communication, Stockholm, August 1974; "Packet Switching Services and the Data Communications User," OVUM, December 1974; and "Adaptive Routing Algorithms for Distributed Computer Neworks", J. McQuillan Thesis, Harvard 1974.
The following definitions of certain terms used herein are included for a more complete understanding of the description to follow. Additional definitions are included throughout the following specification at various points therein for a more complete understanding of the description associated therewith.
A facsimile device is a transceiver including a scanner and printer used for transmitting and receiving documents using analog modulation/demodulation (amplitude and frequency modulation) or digital techniques for converting the graphic and/or alphanumeric information on the document into a data stream, and reconverting such received data from other facsimile devices back into documents.
A teletypewriter is a device for generating coded signals corresponding to a typed alphanumeric character and for typing such alphanumeric characters upon receipt of such coded signals over a communications line.
Visual display unit, or CRT, refers to a data transceiving terminal having a keyboard and cathode ray tube or other visual display means for sending and receiving messages over a communication line and displaying such messages on the visual display means.
Word processing terminal is a typewriter having a data buffering, communications and memory capability utilized in an office environment.
A communications channel is defined as the transmission link between the various data sources and destination, and includes high speed digital data lines at 56 K-bps, 230 K-bps, or other facilities at different data rates; intercity and intracity telephone and telecommunications lines, both analog and digital; microwave transmission with a plurality of voice channels modulated thereon. Such facilities are readily available and provided by a variety of common carriers, together with requisite modems and other interconnecting equipment, and enable each individual local teleprocessing terminal to communicate with any other local or remote teleprocessing terminal via full-duplex channels.
The terms data bytes and data words are used interchangeably herein and are defined as a fixed length binary bit pattern. A bit is defined herein as the smallest unit of binary information.